For reasons of safety, the tire pressure of motor vehicles must be checked regularly and this is often neglected by the driver of the motor vehicle for various reasons. For this reason, tire pressure control systems have already been developed which include a tire pressure control device on each wheel which automatically measures the tire pressure of the tires of the motor vehicle and announces to the driver of the motor vehicle at least a critical deviation from a desired tire pressure. The tire pressure control devices can, for example, be vulcanized into the tires or can be mounted with adhesive or can be attached on or in the valve or on or in the wheel rim. Appropriate configurations are known.
U.S. patent application Ser. No. 09/131,910, filed Aug. 10, 1998, discloses a tire pressure control system wherein a tire pressure control device is assigned to each tire of the motor vehicle. At regular intervals, a tire pressure control device transmits a measured pressure signal together with an individual identifier to a central unit. Because of the transmission of an individual identifier, the situation is avoided that the data, which are transmitted to the central unit, are, for example, mistaken for data which are transmitted by another motor vehicle. In the central unit, value pairs of the form (identifier of the tire pressure control device/wheel position) are stored for each wheel of the motor vehicle so that by a corresponding comparison in the central unit, a conclusion can be drawn as to which identifier with the pressure signal corresponding thereto is transmitted from which wheel position of the motor vehicle. A deviation of the transmitted pressure signal from a pregiven value at a wheel position is displayed to the driver of the motor vehicle by the central unit so that the driver can initiate suitable measures.
The embodiments show that the tire pressure control system, which is known from U.S. patent application Ser. No. 09/131,910, can only function without problems when the allocations (identifier of the tire pressure control device/wheel position) are correctly stored in the central unit. Correspondingly, a new allocation must be undertaken at least after each change of tires. In the above-identified patent application, this new allocation takes place in the known tire pressure control system as follows: each tire pressure control device is assigned an rpm sensor which is switched on for a first time interval. During the first time interval, a first defined angular position of the wheel (to which the rpm sensor is assigned) is determined from the signal of the rpm sensor. The individual identifier is transmitted by the tire control device to the central unit at a first time point t1 at which the wheel assumes this first defined angular position. The same rpm sensor is later switched on for a second time interval during which the same defined angular position of the wheel is determined from the signal of the rpm sensor as in the first interval. The tire pressure control device transmits its individual identifier to the central unit at a second time point t2 in which the wheel assumes this defined angular position. The central unit knows that the wheel (from which the individual identifier has been transmitted) has made an integer number of revolutions between the time points t1 and t2. A check is now made in the central unit as to from which rpm sensor or from which wheel position an integer number of revolutions was transmitted between the time points t1 and t2. The rpm sensors are sensors of a slip control system and are each fixedly mounted at a location. The corresponding wheel position is assigned in the central unit to the individual identifier transmitted from the tire pressure control device. The remaining tire pressure control devices of the motor vehicle are assigned to their wheel positions in the same manner.
A reliable allocation of the tire pressure control devices to the wheel positions in a tire pressure control system of a motor vehicle is possible with the method disclosed in U.S. patent application Ser. No. 09/131,910, filed Aug. 10, 1998. However, the tire pressure control system requires an rpm sensor in every tire pressure control device which drives up the cost of the system. Furthermore, the rpm sensors are a load on the batteries of the tire pressure control devices even though they are switched on only during short time intervals. In this way, the service life of the batteries is reduced so that the high service life of the batteries which is sought is realizable only with difficulty.
A tire pressure control system is disclosed in German patent publication 4,205,911 wherein a new allocation is carried out in that the intensity of the signals (transmitted by the individual tire pressure control devices) is measured by receivers with each receiver being permanently assigned to a corresponding one of the wheel positions and each signal transmitted from a pressure control device is allocated to the wheel position at which the highest signal intensity is generated. For example, the signal intensity of the tire pressure control device, which is located in the forward left tire, will be largest at the receiver which is allocated to the forward left wheel position so that a corresponding allocation can be determined. The respective allocations are stored in the central unit.
For the allocation method explained above, a receiver is needed at each wheel position of the motor vehicle whereby the costs of the tire pressure control system disclosed in German patent publication 4,205,911 are likewise increased.
It is an object of the invention to provide a method for allocating the tire pressure control devices to the wheel positions in a tire pressure control system of a motor vehicle which can be carried out with a cost-effective tire pressure control system.
The method of the invention is in the context of a tire pressure control system of a motor vehicle having a plurality of wheels at respective wheel positions. The method is for allocating tire pressure control devices of the tire pressure control system to the wheel positions of the motor vehicle during the operation of the motor vehicle. The system further includes: a central unit; the tire pressure control devices being assigned to corresponding ones of the wheels; each of the tire pressure control devices functioning to transmit an individual identifier to the central unit at time intervals; a plurality of rpm sensors assigned to corresponding ones of the wheels and the rpm sensors being fixedly mounted at respective ones of the wheel positions with the allocation of the rpm sensors to the wheel positions being known by the central unit; and, the central unit storing the allocation (identifier of the tire pressure control device/wheel position) for each of the wheels; the method including the steps of: causing one of the tire pressure control devices assigned to one of the wheels to transmit to the central unit, at any desired time point, its own individual identifier and a first lengthened high frequency signal, which extends over a first time interval I1; and, as a consequence of a rotation of the one wheel, the signal having an individual trace dependent upon the rotational angle of the one wheel and/or upon the time; causing the one tire pressure control device to transmit to the central unit, at any desired later time point, its own individual identifier and a second lengthened high frequency signal, which extends over a second time interval I2, the second lengthened high frequency signal having the same individual trace at least segmentwise as the first lengthened high frequency signal, the individual trace of the second lengthened high frequency signal being dependent upon time and/or the rotational angle of the one wheel; in the central unit, determining a first time point at which the one wheel assumes a desired relative angular position from the trace of the first lengthened high frequency signal; in the central unit, determining a second time point at which the one wheel assumes the same relative angular position as the one wheel assumed at the first time point and the second time point being determined from the trace of the second lengthened high frequency signal; in the central unit, determining the number of revolutions which the wheels have made in a third time interval I3 from the first time point to the second time point and the number of revolutions for the wheels being determined from the signals of the rpm sensors; in the central unit, checking which one of the wheels in which one of the positions had made an integer number of revolutions; and, in the central unit, assigning the corresponding wheel position to the individual identifier transmitted from the one tire pressure control device.
With respect to the term xe2x80x9clengthened high frequency signalxe2x80x9d, the following is noted. Each tire pressure control device transmits its data (that is, especially its individual identifier and the pressure signal) in the form of high frequency signals. A lengthened high frequency signal is a high frequency signal which is significantly longer than the high frequency signals which are conventionally transmitted by a tire pressure control device. A significantly longer signal is one which is longer by at least one order of magnitude.
Preferably, each lengthened high frequency signal is transmitted by the tire pressure control device at a constant maximum amplitude. The constant maximum amplitude, as a consequence of the rotation of the wheel, defines an individual trace in dependence upon the rotational angle of the wheel and/or on the time. The one wheel is that wheel from which the lengthened high frequency signal is transmitted.
The invention also utilizes the situation that the constant maximum amplitude of a transmitted high frequency signal receives an individual trace which is dependent upon the rotational angle of the wheel and/or on the time so that a conclusion can be drawn based on the signal trace as to the relative angular position of the wheel. The individual trace is probably caused by the different geometric characteristics in the wheel boxes of the motor vehicle and by the time-dependent change of the spacing of a tire pressure control device to the wheel box as the device rotates relative to the wheel box.
The advantages achieved with the invention are especially seen in that, for carrying out the method, neither a receiver at each wheel position (as in German patent publication 4,205,911) nor an rpm sensor in each tire pressure control device (as in the above-mentioned U.S. patent application Ser. No. 09/131,910) is needed. Rather, for carrying out the invention, no components are needed which are otherwise present in the tire pressure control system or in the vehicle (such as the rpm sensors, which are components of a slip control system). In this way, the allocation method is carried out with the aid of a cost-effective tire pressure control system. Further advantages of the invention are seen in that the allocation system operates as reliably as the allocation methods known to date and the batteries of the tire pressure control devices are loaded only slightly.
According to another feature of the invention, each time interval during which a lengthened high frequency signal is transmitted from a wheel is at least so long that the wheel makes at least one rotation in the time interval. The advantage of this embodiment is that the first and second lengthened high frequency signals, which are transmitted from a tire pressure control device, have perforce over some regions the same time-dependent trace from which a conclusive angular position can be determined.
According to another feature of the invention, a specific time duration is pregiven in each tire pressure control device which is the same for all lengthened high frequency signals transmitted from this tire pressure control device. Preferably, the time duration is so dimensioned that the probability is very high that the wheel makes a whole revolution during the transmission of the lengthened high frequency signal or the speed of rotation is very low for the wheel to which this condition applies. In this case, it is given with high probability that the first and second lengthened high frequency signals transmitted by a tire pressure control device are coincident at least over some regions with each other. The advantage of this embodiment is that it is easily realized.
According to another embodiment of the invention, each one of the tire pressure control devices has a centrifugal force sensor, which generates a signal above a pregiven number of revolutions of the wheel to which the one tire pressure control device is assigned; and, the one tire pressure control device transmits a lengthened high frequency signal only when the signal of the centrifugal force sensor is present.
The length is pregiven in dependence upon the pregiven number of resolutions at which the centrifugal force sensor generates a signal so that it is ensured that the wheel makes at least one whole revolution during the transmission of the lengthened high frequency signal. Thus, if the centrifugal force sensor generates the signal at x revolutions per minute, then a time of at least 1/x minutes is pregiven for the length of the lengthened high frequency signal. The advantage of this embodiment is that the first and second high frequency signal transmitted by a tire pressure control device are coincident with certainty at least over some regions.
According to still another embodiment of the invention, the tire pressure control system has a central transmitter and each of the tire pressure control devices has a receiver; and, the method includes the further steps of: the central transmitter transmitting a signal to all of the tire pressure control devices; and, directly after receiving the signal, each of the tire pressure control devices transmitting a lengthened high frequency signal to the central unit.
The high frequency signal is transmitted by the tire pressure control devices. Preferably, the central transmitter additionally sends data as to the length of this lengthened high frequency signal to the tire pressure control devices. A length which is dependent upon the vehicle speed is preferably transmitted and this length is so dimensioned that it is ensured that the lengthened high frequency signals are at least so long that each wheel, from which a lengthened high frequency signal is transmitted, makes at least one revolution during the transmission and, in this way, the advantages already mentioned above are attained.
According to another feature of the invention, the allocation method is started by actuating a switch. The switch is preferably actuated by the driver of the motor vehicle. The advantage of this embodiment is that the allocation method is only carried out when it is necessary, for example, after a tire has been exchanged.
According to still another embodiment of the invention, the allocation method is started automatically after switching on the ignition of the motor vehicle when the ignition had been previously switched off for a pregiven time span. The pregiven time span is selected so that an exchange of tires could have taken place in this time which would make a new allocation necessary, that is, approximately 10 to 30 minutes. The advantage of this embodiment is that the method is carried out automatically only when a new allocation could be necessary.
In another embodiment of the invention, the signals, which are transmitted by the tire pressure control devices, are amplitude modulated and each lengthened high frequency signal is a separate signal which is transmitted at a constant maximum amplitude.
According to another embodiment of the invention, the signals which are transmitted by the tire pressure control devices, are frequency modulated and are transmitted at constant maximum amplitude. The individual identifier is contained in the lengthened high frequency signal.
According to still another embodiment of the invention, the first and second time points at which the wheel assumes a coincident relative angular position is determined by cross correlation in the first and second lengthened high frequency signals. In cross correlation, a conventional mathematical method is used as will be described hereinafter. The advantage of this additional embodiment is that a coincident relative angular position can be reliably found in the lengthened high frequency signal in a simple manner.